A study of twins has found that sleeping for less than seven hours a night was linked to higher bodyweights and a greater susceptibility to genetic factors that influence weight.

However the opposite was true in people who slept for nine hours or more.

Several genes have been found to be associated with obesity and this is thought to be the first study to examine how sleep interacts with them.

The genes affect how the body uses energy, how fat is stored, the feeling of being full after a meal and how quickly sugar is used up.

A team at the University of Washington studied 1,088 pairs of twins and found that the genetic influence on their body mass index was twice as great in those who slept for less than seven hours compared to those who slept for nine hours a night.